1. Field of the Invention
The present invention relates generally to a control device for an automatic transmission for a vehicle, and more particularly to a control device for an automatic transmission for a vehicle for performing the slip control of a lock-up clutch at a predetermined gear position before kickdown.
2. Description of the Related Art
In general, an automatic transmission having a torque converter includes a lock-up clutch for directly connecting an output shaft of an engine and an input shaft of the automatic transmission in the condition where the gear position of the automatic transmission is a predetermined gear position and the rotational speed of the engine is greater than or equal to a predetermined rotational speed, in order to improve the fuel economy. A lock-up clutch engagement characteristic line is set in a shift map, and the lock-up clutch is controlled to be engaged at a vehicle speed higher than that corresponding to the lock-up clutch engagement characteristic line. In the case of kickdown, the lock-up clutch is controlled to be disengaged at the time the accelerator pedal angle becomes larger than that corresponding to the lock-up clutch engagement characteristic line. Further, in the case of running on an uphill road, the lock-up clutch engagement characteristic line is shifted toward higher vehicle speeds to limit the engagement of the lock-up clutch in a high vehicle speed region.
As mentioned above, the lock-up clutch engagement characteristic line is fixedly set in the shift map in the prior art. Accordingly, in the case that the lock-up clutch engagement characteristic line is shifted toward higher vehicle speeds to reduce the range of the lock-up region, smooth running can be attained. However, the fuel consumption is increased. Conversely, in the case that the range of the lock-up region is set wide, the fuel consumption can be reduced. However, smooth running becomes difficult to attain in this case. Thus, it is difficult to attain both the reduction in fuel consumption and the improvement in drivability.